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The new genus, Paraleptosphaeroma, is distinguished from the morphologically similar genus Leptosphaeroma by the fusion of the pleonites, the shape of the peni, the attachment of appendix masculina, and the presence or the absence of an endopod on the first pleopod. The term cingula is introduced to refer to lateral margins fringed with setae and the distinction is drawn between setal and membranous forms of the cingula. Paraleptsphaeroma glynni (sp nov.) occurs on cobble in tidal pools and is associated with three sessile species, two bryozoans and a coralline alga. Experiments demonstrate the isopod to be a sequential hermaphrodite with a socially mediated sexual transformation. Brood mortality is very low. Observations of feeding and analysis of gut contents show anascan bryozoans to be the principal food sources of P. glynni. The principal color polymorphisms of juveniles are illustrated. The apparent concordance between these patterns and those produced by the boring activities of phoronids and barnacles is tested, with results suggesting that the color patterns are adaptations to concealment from fish predators. This interpretation is further supported by morphological evidence and experiments demonstrating the poor swimming ability of P. glynni.